Field of the Invention
This invention relates in general to electronic imaging apparatus and in particular, to an electronic still camera that employs a non-volatile reuseable storage medium for recording scene images.
Description of the Prior Art
Conventional cameras employ a shutter for exposing for a limited duration photographic film located at a film exposure plane. Film processing solutions are required to produce a visible image of trhe photographed scene. As is well known, processed photographic film may not be reused.
Recently, considerable effort has been given to the development of solid-state elements for imaging purposes. Such elements offer an advantage over photographic film in that, theoretically, they can be reused any number of times for imaging.
A solid-state imaging element of the type known as a charge transfer device can include a semiconductive substrate doped with majority carriers and covered with a thin insulating film upon which is located an array of electrically conductive electrodes. A depletion region or potential well is formed within the semiconductor under electrodes biased by a voltage of the proper polarity. The use of a charge transfer device involves the basic concept of forming a charge pattern consisting of packets of minority carriers in the potential wells. In the course of an optical imaging process, photons incident upon the semiconductor generate minority carriers within these potential wells in proportion to the amount of light impinging upon the semiconductor in the near vicinity of each well. These packets can be transferred through the semiconductor by sequentially biasing the electrodes. The potential wells effectively "move" through the semiconductor sweeping the minority carriers along within the wells. At an appropriate location these packets may be detected, for example, by removing them sequentially from the semiconductor by means of a reverse-biased diode coupled to transfer electrodes. An arrangement for read-out of information from a charge transfer device is disclosed in IEEE Transactions On Electron Devices, Vol. ED-20, No. 6, June 1973, in an article entitled "Interlacing In Charge Coupled Imaging Devices", by Carlo H. Sequin.
As with conventional photographic film, to obtain a scene image having a proper contrast, the imaging element must not be overexposed, or, in other words, the potential wells must not be saturated with minority carriers during the exposure interval. However, even in the absence of illumination, the regions constituting the potential wells tend to saturate with the passage of time by means of the thermal generation of minority carriers. Carriers generated in this manner constitute an unwanted signal commonly known as a "dark current". It is important that this signal be only a small fraction of the signal produced by incident illumination, particularly if the dark current is non-uniform over the imaging area.
U.K. Pat. No. 1,440,792, entitled ELECTRONIC STILL PICTURE CAMERA, and U.S. Pat. No. 4,057,830 which corresponds thereto and is entitled Electronic Photography System, disclose a camera for electronically recording "stop-action" or still pictures that includes a charge transfer device, and recording apparatus that employs an inexpensive information-recording medium which is non-volatile and reuseable, such as a magnetic tape, disc or drum. The camera also includes a conventional shutter mechanism for exposing the transfer device to reflected scene light for a duration related to scene brightness. The rate of read-out of the signals produced by the charge transfer device is synchronized with the speed of the recording apparatus since the transfer device output is connected directly to the input of the recording apparatus. These signals are read-out at a relatively slow speed to record a scene image on the storage medium that is employed. It takes approximately one second to output scene information from the charge transfer device. For "stop-action" photography an exposure interval of approximately 1/20 second or less is needed. Accordingly, the charge transfer device is used both for imaging and until scene information is read-out, for data storage.
That camera suffers from the disadvantage that the charge pattern related to the incident illumination will be adversely affected by thermally generated minority carriers. Not only would it be expected that significant dark current would be produced with a 1-second storage interval, but it could also be expected that the "dark current" would be nonuniform. This is because the storage interval for any potential well, and accordingly the number of thermally generated minority carriers in that well, is dependent upon whether or not that potential well is among the first or the last to sweep through the semiconductor to an output transfer electrode. Furthermore, saturation of some potential wells may occur if too many minority carriers are thermally generated. Excess minority carriers would spread to adjacent potential wells to be added to minority carriers in non-saturated potential wells.